In the event that the welder was to stop welding and all functions of the machine stopped simultaneously then the consumable filler wire would in all likelihood freeze in the weld pool. In order to avoid this happening the burn back feature is present on most MIG welders.
This facility may be built in or have an adjustable control. It will allow the power and gas shield to be maintained on the consumable filler wire when it has stopped feeding thereby burning clear of the weld.
In some equipment the burn back is preset within the control circuits others offer an external variable control feature to adjust the time of delay.